A solid-state semiconductor device usually comprises a wafer of semiconductor material in which doped P type and N type regions form one or more PN junctions. Typically, several of these PN junctions intercept a surface of the wafer. For example, in what is commonly referred to as a planar semiconductor device, ie., a device wherein P type and/or N type conductivity regions are diffused into the semiconductor from selected portions of a major wafer surface, the PN junction associated with each of the diffused regions intercepts the major wafer surface.
When a semiconductor device is used in a power control application, certain of the PN junctions therein must support relatively high voltages during device operation. Such PN junctions are typically referred to as high voltage junctions, and it is desirable that they be able to support voltages which approach the theoretical bulk breakdown value associated therewith. The theoretical bulk breakdown voltage of a PN junction is a function of the doping profiles in the P type and N type regions adjacent thereto and the geometry of the junction.
The effect of PN junction geometry on breakdown voltage is quite apparent, for example, in planar double-diffused bipolar power transistors. So as to spread the electric field associated with the high voltage base/collector PN junction of such devices, one approach has been to incorporate a relatively lightly doped epitaxial layer, of similar conductivity type to the base region, between the base and collector regions. Such an approach, as described in HIGH VOLTAGE THIN LAYER DEVICES (RESURF DEVICES), J. A. Appels et al., IEDM Technical Digest, 1979, pp. 238-241, essentially renders the base/collector PN junction plane in shape. However, there is a drawback to this prior art RESURF structure in that the additional epitaxial layer effectively widens the device base region. This in turn decreases device switching speed in that the limiting frequency, f.sub.T, is proportional to the inverse square of the width of the base region between the emitter and collector regions.